Experimental study on the effects of microplastics and surfactants on ocean surface roughness

High correlations between ocean surface roughness and the presence of floating microplastics are recently detected by the NASA CYGNSS satellites. In order to understand the mechanisms underlying the correlation, we conduct wave tank experiments to study the effects of surface particles and surfactants on surface roughness (as two hypothetical mechanisms). In a 35-meter wave tank, we compare the surface roughness (measured by mean square slope, MSS) of irregular waves generated by a mechanical wave maker with and without the presence of particles/surfactants. For particles, we find that MSS is increased compared to the clean-water situation with low concentration of particles (probably due to the diffracted waves). With the increasing concentration, the MSS gradually decreases and is eventually subject to a damping effect (relative to clean-water MSS). For surfactants, we observe significant damping of MSS, which is explained through the Marangoni damping effect.